The conventional processes for producing various types of biofuels, such as alcohol and other chemicals, from grains generally follow similar procedures. Wet mill processing plants convert, for example, corn grain, into several different co-products, such as germ (for oil extraction), gluten feed (high fiber animal feed), gluten meal (high protein animal feed) and starch-based products such as alcohol (e.g., ethanol or butanol), high fructose corn syrup, or food and industrial starch. Dry grind plants generally convert grains, such as corn, into two products, namely alcohol (e.g., ethanol or butanol) and distiller's grains with solubles. If sold as wet animal feed, distiller's wet grains with solubles are referred to as DWGS. If dried for animal feed, distiller's dried grains with solubles are referred to as DDGS. This co-product provides a secondary revenue stream that offsets a portion of the overall alcohol production cost.
With respect to the wet mill process, FIG. 1 is a flow diagram of a typical wet mill alcohol (e.g., ethanol) production process 10. The process 10 begins with a steeping step 12 in which grain (e.g., corn) is soaked for 24 to 48 hours in a solution of water and sulfur dioxide in order to soften the kernels for grinding, leach soluble components into the steep water and loosen the protein matrix with the endosperm. Corn kernels contain mainly starch, fiber, protein and oil. The mixture of steeped corn and water is then fed to a degermination mill step (first grinding) 14 in which the corn is ground in a manner that tears open the kernels and releases the germ so as to make a heavy density (8.5 to 9.5 Be) slurry of the ground components, primarily a starch slurry. This is followed by a germ separation step 16 that occurs by flotation and use of a hydrocyclone(s) to separate the germ from the rest of the slurry. The germ is the part of the kernel that contains the oil found in corn. The separated germ stream, which contains some portion of the starch, protein and fiber, goes to germ washing to remove starch and protein, and then to a dryer to produce about 2.7 to 3.2 Lb. (dry basis) of germ per bushel of corn. The dry germ has about 50% oil content on a dry basis.
The remaining slurry, which is now devoid of germ, but containing fiber, gluten (i.e., protein) and starch, is then subjected to a fine grinding step (second grinding) 20 in which there is total disruption of endosperm and release of endosperm components, namely gluten and starch, from the fiber. This is followed by a fiber separation step 22 in which the slurry is passed through a series of screens in order to separate the fiber from starch and gluten and to wash the fiber clean of gluten and starch. The fiber separation stage 22 typically employs static pressure screens or rotating paddles mounted in a cylindrical screen (Paddle Screens). Even after washing, the fiber from a typical wet grind mill contains 15 to 20% starch. This starch is sold with the fiber as animal feed. The remaining slurry, which is now generally devoid of fiber, is subjected to a gluten separation step 24 in which centrifugation or hydrocyclones separate starch from the gluten. The gluten stream goes to a vacuum filter and dryer to produce gluten (protein) meal.
The resulting purified starch co-product then can undergo a jet cooking step 26 to start the process of converting the starch to sugar. Jet cooking refers to a cooking process performed at elevated temperatures and pressures, although the specific temperatures and pressures can vary widely. Typically, jet cooking occurs at a temperature of about 93 to 110° C. (about 200 to 230° F.) and a pressure of about 30 to 50 psi. This is followed by liquefaction 28, saccharification 30, fermentation 32, yeast recycling 34, and distillation/dehydration 36 for a typical wet mill biofuels system. Liquefaction occurs as the mixture or “mash” is held at 90 to 95° C. in order for alpha-amylase to hydrolyze the gelatinized starch into maltodextrins and oligosaccharides (chains of glucose sugar molecules) to produce a liquefied mash or slurry. In the saccharification step 30, the liquefied mash is cooled to about 50° C. and a commercial enzyme known as gluco-amylase is added. The gluco-amylase hydrolyzes the maltodextrins and short-chained oligosaccharides into single glucose sugar molecules to produce a liquefied mash. In the fermentation step 32, a common strain of yeast (Saccharomyces cerevisae) is added to metabolize the glucose sugars into ethanol and CO2.
Upon completion, the fermentation mash (“beer”) will contain about 15% to 18% ethanol (volume/volume basis), plus soluble and insoluble solids from all the remaining grain components. The solids and some liquid remaining after fermentation go to an evaporation stage where yeast can be recovered as a byproduct. Yeast can optionally be recycled in a yeast recycling step 34. In some instances, the CO2 is recovered and sold as a commodity product. Subsequent to the fermentation step 32 is the distillation and dehydration step 36 in which the beer is pumped into distillation columns where it is boiled to vaporize the ethanol. The ethanol vapor is separated from the water/slurry solution in the distillation columns and alcohol vapor (in this instance, ethanol) exits the top of the distillation columns at about 95% purity (190 proof). The 190 proof ethanol then goes through a molecular sieve dehydration column, which removes the remaining residual water from the ethanol, to yield a final product of essentially 100% ethanol (199.5 proof). This anhydrous ethanol is now ready to be used for motor fuel purposes. Further processing within the distillation system can yield food grade or industrial grade alcohol.
No centrifugation step is necessary at the end of the wet mill ethanol production process 10 as the germ, fiber and gluten have already been removed in the previous separation steps 16, 22, 24. The “stillage” produced after distillation and dehydration 36 in the wet mill process 10 is often referred to as “whole stillage” although it also is technically not the same type of whole stillage produced with a traditional dry grind process described in FIG. 2 below, since no insoluble solids are present. Other wet mill producers may refer to this type of stillage as “thin” stillage.
The wet grind process 10 can produce a high quality starch product for conversion to alcohol, as well as separate streams of germ, fiber and protein, which can be sold as co-products to generate additional revenue streams. However, the overall yields for various co-products can be less than desirable and the wet grind process is complicated and costly, requiring high capital investment as well as high-energy costs for operation.
Because the capital cost of wet grind mills can be so prohibitive, some alcohol plants prefer to use a simpler dry grind process. FIG. 2 is a flow diagram of a typical dry grind alcohol (e.g., ethanol) production process 100. As a general reference point, the dry grind method 100 can be divided into a front end and a back end. The part of the method 100 that occurs prior to distillation 110 is considered the “front end,” and the part of the method 100 that occurs after distillation 110 is considered the “back end.” To that end, the front end of the dry grind process 100 begins with a grinding step 102 in which dried whole corn kernels can be passed through hammer mills for grinding into meal or a fine powder. The screen openings in the hammer mills or similar devices typically are of a size 6/64 to 9/64 inch, or about 2.38 mm to 3.57 mm, but some plants can operate at less than or greater than these screen sizes. The resulting particle distribution yields a very wide spread, bell type curve, which includes particle sizes as small as 45 micron and as large as 2 to 3 mm. The majority of the particles are in the range of 500 to 1200 micron, which is the “peak” of the bell curve.
After the grinding step 102, the ground meal is mixed with cook water to create a slurry at slurry step 103 and a commercial enzyme called alpha-amylase is typically added (not shown). The slurry step 103 is followed by a liquefaction step 104 whereat the pH is adjusted to about 5.2 to 5.8 and the temperature maintained between about 50° C. to 105° C. so as to convert the insoluble starch in the slurry to soluble starch. Various typical liquefaction processes, which occur at this liquefaction step 104, are discussed in more detail further below. The stream after the liquefaction step 104 has about 30% dry solids (DS) content, but can range from about 29-36%, with all the components contained in the corn kernels, including starch/sugars, protein, fiber, starch, germ, grit and oil and salts, for example. Higher solids are achievable, but this requires extensive alpha amylase enzyme to rapidly breakdown the viscosity in the initial liquefaction step. There generally are several types of solids in the liquefaction stream: fiber, germ and grit.
Liquefaction may be followed by separate saccharification and fermentation steps, 106 and 108, respectively, although in most commercial dry grind ethanol processes, saccharification and fermentation can occur simultaneously. This single step is referred to in the industry as “Simultaneous Saccharification and Fermentation” (SSF). Both saccharification and SSF can take as long as about 50 to 60 hours. Fermentation converts the sugar to alcohol. Yeast can optionally be recycled in a yeast recycling step (not shown) either during the fermentation process or at the very end of the fermentation process. Subsequent to the fermentation step 108 is the distillation (and dehydration) step 110, which utilizes a still to recover the alcohol.
Finally, a centrifugation step 112 involves centrifuging the residuals produced with the distillation and dehydration step 110, i.e., “whole stillage” in order to separate the insoluble solids (“wet cake”) from the liquid (“thin stillage”). The liquid from the centrifuge contains about 5% to 12% DS. The “wet cake” includes fiber, of which there generally are three types: (1) pericarp, with average particle sizes typically about 1 mm to 3 mm; (2) tricap, with average particle sizes about 500 micron; (3) and fine fiber, with average particle sizes of about 250 micron. There may also be proteins with a particle size of about 45 to 300 micron.
The thin stillage typically enters evaporators in an evaporation step 114 in order to boil or flash away moisture, leaving a thick syrup which contains the soluble (dissolved) solids (mainly protein and starches/sugars) from the fermentation (25 to 40% dry solids) along with residual oil and fine fiber. The concentrated slurry can be sent to a centrifuge to separate the oil from the syrup in an oil recovery step 116. The oil can be sold as a separate high value product. The oil yield is normally about 0.6 lb./bu of corn with high free fatty acids content. This oil yield recovers only about ⅓ of the oil in the corn, with part of the oil passing with the syrup stream and the remainder being lost with the fiber/wet cake stream. About one-half of the oil inside the corn kernel remains inside the germ after the distillation step 110, which cannot be separated in the typical dry grind process using centrifuges. The free fatty acids content, which is created when the oil is heated and exposed to oxygen throughout the front and back-end process, reduces the value of the oil. The (de-oil) centrifuge only removes less than 50% because the protein and oil make an emulsion, which cannot be satisfactorily separated.
The syrup, which has more than 10% oil, can be mixed with the centrifuged wet cake, and the mixture may be sold to beef and dairy feedlots as Distillers Wet Grain with Solubles (DWGS). Alternatively, the wet cake and concentrated syrup mixture may be dried in a drying step 118 and sold as Distillers Dried Grain with Solubles (DDGS) to dairy and beef feedlots. This DDGS has all the corn and yeast protein and about 75% of the oil in the starting corn material. But the value of DDGS is low due to the high percentage of fiber, and in some cases the oil is a hindrance to animal digestion and lactating cow milk quality.
Further with respect to the liquefaction step 104, FIG. 3 is a flow diagram of various typical liquefaction processes that define the liquefaction step 104 in the dry grind process 100. Again, the dry grind process 100 begins with a grinding step 102 in which dried whole corn kernels are passed through hammer mills or similar milling systems such as roller mills, flaking mills, impacted mill or pin mills for grinding into meal or a fine powder. The grinding step 102 is followed by the liquefaction step 104, which itself includes multiple steps as is discussed next.
Each of the various liquefaction processes generally begins with the ground grain or similar material being mixed with cook and/or backset water, which can be sent from evaporation step 114 (FIG. 2), to create a slurry at slurry tank 130 whereat a commercial enzyme called alpha-amylase is typically added (not shown). The pH is adjusted here, as is known in the art, to about 5.2 to 5.8 and the temperature maintained between about 50° C. to 105° C. so as to allow for the enzyme activity to begin converting the insoluble starch in the slurry to soluble liquid starch. Other pH ranges, such as from pH 4.0-7.0, may be utilized and an acid treatment system using sulfuric acid, for example, can be used as well for pH control and conversion of the starches to sugars.
After the slurry tank 130, there are normally three optional pre-holding tank steps, identified in FIG. 3 as systems A, B, and C, which may be selected depending generally upon the desired temperature and holding time of the slurry. With system A, the slurry from the slurry tank 130 is subjected to a jet cooking step 132 whereat the slurry is fed to a jet cooker, heated to about 120° C., held in a U-tube or similar holding vessel for about 2 to about 30 minutes, then forwarded to a flash tank. In the flash tank, the injected steam flashes out of the liquid stream, creating another particle size reduction and providing a means for recovering the injected stream. The jet cooker creates a sheering force that ruptures the starch granules to aid the enzyme in reacting with the starch inside the granule and allows for rapid hydration of the starch granules. It is noted here that system A may be replaced with a wet grind system. With system B, the slurry is subjected to a secondary slurry tank step 134 whereat the slurry is maintained at a temperature from about 90° C. to 100° C. for about 10 min to about one hour. With system C, the slurry from the slurry tank 130 is subjected to a secondary slurry tank—no steam step 136, whereat the slurry from the slurry tank 130 is sent to a secondary slurry tank, without any steam injection, and maintained at a temperature of about 80° C. to 90° C. for about 1 to 2 hours. Thereafter, the slurry from each of systems A, B, and C is forwarded, in series, to first and second holding tanks 140 and 142 for a total holding time of about 60 minutes to about 4 hours at temperatures of about 80° C. to 90° C. to complete the liquefaction step 104, which then is followed by the saccharification and fermentation steps 106 and 108, along with the remainder of the process 100 of FIG. 2. While two holding tanks are shown here, it should be understood that one holding tank, more than two holding tanks, or no holding tanks may be utilized.
In today's typical grain to biofuel plants (e.g., corn to alcohol plants), many systems, particularly dry grind systems, process the entire corn kernel through fermentation and distillation. Such designs require about 30% more front-end system capacity because there is only about 70% starch in corn, with less for other grains and/or biomass materials. Additionally, extensive capital and operational costs are necessary to process the remaining non-fermentable components within the process. By removing undesirable, unfermentable components prior to fermentation (or other reaction process), more biofuel, biochemical and other processes become economically desirable.
It thus would be beneficial to provide an improved dry milling method and system that produces a cleaner sugar stream, such as for biofuel production, that may be similar to the sugar stream produced by conventional wet corn milling systems, but at a fraction of the cost and generate additional revenue from oil, protein and/or fiber yields, for example.